This invention relates generally to perforating apparatus and methods and more particularly, but not by way of limitation, to a design for such apparatus and methods by which these are implemented to incorporate spirally distributed charges held in place by respective retainer clips.
One means for facilitating the flow of an oil or gas well includes perforating the formation and any casing or lining adjacent the formation. To perforate, a perforating gun, loaded with explosive charges, is lowered into the well to the desired depth on a tubing string or a wireline as known to the art. A perforating gun generally has an elongated tubular configuration, and the charges are frequently cup-like members holding conically constrained explosive material. Once lowered into the well on the tool, the charges are ignited and fired into the formation in a known manner.
The spatial distribution of the charges on the gun is an important consideration in designing a particular type of perforating gun and in designing a particular perforation job because this design affects the integrity of the gun and the ability of the gun to produce effective perforations. As to the integrity of the gun, the distribution of the charges affects the shock load applied to the gun structure upon a detonation of one or more of the charges. For example, a gun having a known distribution using three charges clustered on a common transverse plane of the gun and fired by a common detonator cord receives a greater shock load than a gun having a similarly sized single charge per transverse plane because in the former the three charges are fired simultaneously whereas in the latter only one is fired at a time. The distribution of the charges also affects the collapse strength of the gun. For example, in a gun having an outer body along which are formed scallops through which the charges are fired, the gun will more readily collapse (and thus have a weaker collapse strength) when longitudinally aligned ones of the scallops are longitudinally closer together. Thus, from the integrity of the gun standpoint, a more durable gun is one having the fewest number of charges per transverse plane to be ignited at one time and having the greatest spacing between longitudinally aligned charges or scallops in a scalloped body. These two particular design factors considered alone, however, may not produce the most effective perforating job.
To produce an effective perforating job, the perforations of course need to enter the desired zone. Although longitudinal spacing between charges is only a few inches at most in present perforating guns, such seemingly close spacing can constitute a sufficiently great distance that certain oil or gas containing formations are missed. These formations are very thin strata known as lensatic or laminated formations. Thus, the longitudinal spacing needs to be relatively close when the gun is to be used to perforate such a thin formation.
To produce an effective perforating job in a highly deviated hole traversing an unconsolidated formation, the charges need to be distributed so that a majority of the load is fired in one of two general directions to prevent damaging, rather than improving, the flowability of the formation. That is, in a highly deviated hole traversing an unconsolidated formation, the majority of the charges are preferably fired in a downward direction when the perforating occurs in a relatively horizontal portion of the deviated hole because if equal amounts are fired upward and downward, or if a majority is fired upward, the unconsolidated formation might collapse onto the gun and clog the well bore.
Still another factor related to the effectiveness of the perforating job is the distance of the apex of a conically shaped charge from the first obstacle or target through which the charge is to be fired. This distance is referred to as the "standoff." In a gun having the aforementioned scalloped outer body, the first obstacle or target is the body wall defining the bottom of the scallop with which the respective charge is radially aligned. The spacing between the charge's apex and this wall is important because the greater the spacing, the better formed is the explosive jet generated by the detonation of the charge. The better formed the jet is, the better the resultant perforation will likely be.
These factors are known to the art, and perforating guns which to some degree meet one or more of them are known. One type of gun has a single spiral distribution of charges along a given length of the gun. This has a relatively great strength against collapse because any longitudinally aligned charges are spaced a distance equal to or greater than the given length along which this single spiral occurs. This design also creates a relatively small shock load because only one charge is located on a transverse plane and fired at any one time. This design can be adapted to have an adequate standoff and to provide for directional firing of a majority of the charges. This design, however, is relatively poor at producing effective perforations. An example of this type of distribution is illustrated in FIG. 1(a).
Another type of distribution, previously alluded to as a cluster type, has multiple charges located on a common transverse plane with adjacent sets of clusters longitudinally spaced by a few inches and circumferentially offset. This offset is referred to as "phasing," and in one specific configuration known to us is 60.degree.. In this specific design, a first cluster of three charges is positioned at a first transverse plane of the gun with each of the three charges angularly spaced from the next by 120.degree.. A second cluster of three charges is located in a second transverse plane three inches below the first transverse plane. The three charges of this second cluster are spaced 120.degree. from each other, but each of these charges is also offset 60.degree. from the longitudinal plane containing the center of a respective one of the charges in the first cluster. A third cluster is spaced three inches below the second, but longitudinally aligned with the first cluster and thereby offset 60.degree. from the second cluster. A fourth cluster is spaced three inches below the third, but longitudinally aligned with the second set. This distribution, illustrated in FIG. 1(b), has a relatively good collapse strength (not as good as the first mentioned design, however), but it has a relatively high shock load because each charge within a cluster is fired simultaneously with the other two. This design yields an improved perforating performance over the first mentioned design, but it cannot be readily adapted to improve standoff because the three charges abut each other, thereby preventing further transverse displacement of a charge away from the outer wall of the gun. Furthermore, the illustrated three-charge cluster design has an even distribution of charges which prevents focusing a majority of them in a single general direction. It also is somewhat limited in how longitudinally close together the clusters can be set so that this design is not an optimum one for effectively perforating very thin formations.
Still another type of distribution design is a 120.degree. phasing, multiple spiral pattern, which is illustrated in FIG. 1(c). This design has one charge per transverse plane with subsequent charges longitudinally spaced one inch below and 120.degree. from the preceding one. This design has the poorest collapse strength of the three mentioned designs, but it has a lower shock load than the second-mentioned design. This third design produces the best perforating array of the three, and it can fire a majority of its charges in one general direction. By having only a single charge per transverse plane, it is adaptable for improving its standoff.
Although there are perforating guns which perform satisfactorily in particular uses, there is the need for an improved design for a gun which would be adaptable to satisfy within a single gun all of the aforementioned factors of shock load, collapse strength, directional firing of a majority of the charges, increased standoff, and better shot coverage in an overall combination which none of the aforementioned designs individually satisfies.